1. Field of the Invention
The present invention relates to an optical distance measuring apparatus realizing, with a quite high resolution, measurement of a profile of a surface state of an object under measurement, measurement or observation of a surface state and an internal state of a cell or the like by irradiation of laser lights, and is preferable for an apparatus improving a resolution of an optical apparatus such as a microscope.
2. Description of the Related Art
With a conventional optical microscope, it has been difficult to perform three-dimensional measurement, and besides, it has not been possible to observe or measure an object under measurement at or below a diffraction limit. As a substitute for this, apparatuses such as a scanning electron microscope, a probe microscope (STM, AFM, NFOS, or the like), and a confocal microscope have been developed and used in many fields.
The scanning electron microscope uses a very narrow beam as a scanning electron probe, and thus has a high resolution and a significantly large focal depth as compared to the optical microscope. However, for observing an object under measurement with low electric conductivity such as a cell, it is necessary to coat platinum palladium or gold with good electric conductivity on a sample as the object under measurement. Accordingly, this often accompanies damage to a cell itself, and of course it has not been possible to observe or measure a live cell.
Further, the probe microscope is to measure a distance to an object under measurement by making a probe, which is disposed close to the object under measurement, further close to the object under measurement, and utilizing atomic force, tunnel current, light near field or the like. However, it is difficult to move the probe at high speed, handling is difficult because the distance to the object under measurement is quite close, and moreover a long time is needed for obtaining two-dimensional information.
On the other hand, the confocal microscope irradiates a spot light to an object under measurement, and moves an objective lens or the object under measurement so that an amount of light received by a photo detector disposed at a confocal position becomes maximum for this spot light via a pin hole, to thereby obtain height information or path difference information of the object under measurement. However, in the confocal microscope, basically, when there is a phase distribution within a spot, the beam is deformed, which results in erroneous information. In particular, for an object under measurement on which a wavefront changes in phase, like a refractive index change in a cell or the like, it must be said that reliability of the value thereof is poor. Further, it is necessary to move the objective lens or the object under measurement so that the received amount of light becomes maximum, and thus the measurement cannot be performed in real time.
Under such circumstances, in accordance with development of micronanotechnology field in recent years, a technique of measuring, at high speed, three-dimensional information of a minute industrial product or precision component, is drawing attention. In addition to this, there is increasing a demand such that, in biology, medical science, and agricultural science, three-dimensional profile information regarding a biological sample having a thickness such a cell, in a living state, is wanted to be obtained in real time.
On the other hand, as one of methods for measuring a distance and a thickness with high accuracy or for measuring or observing a minute object with high accuracy by using a microscope, a heterodyne interference method is well known. Here, an optical heterodyne method using lights will be described, but, a heterodyne method is also performed with the similar idea for other electromagnetic waves. This optical heterodyne method makes two laser lights with different frequencies interfere with each other to create a beat signal of the frequency difference, and detects a phase change of this beat signal with a resolution of about 1/500 of a wavelength. That is, with this optical heterodyne method, it is possible to measure a distance to an object under measurement while measuring a change in a height direction of a surface being three-dimensional information, or to measure or observe a thickness and the like of an object under measurement itself.
Patent Document 1: Japanese Patent Application Laid-open No. S59-214706
The aforementioned Japanese Patent Application Laid-open No. S59-214706 of Patent Document 1 discloses a method to adjacently generate two beams composed of different wavelengths by using an acoustic optical device, detect a phase change between these two beams, and obtain a surface profile by increasing the phase change cumulatively. However, this Patent Document 1 is to make two beams be close and slightly larger than a beam profile, detect an average phase difference in two beam profiles by heterodyne wave detection, and sequentially integrate the phase difference, so as to obtain concave and convex information.
Therefore, according to this Patent Document 1, it is possible to measure concave and convex information of an object under measurement which is assumed to be flat such as a semiconductor wafer, but it is not possible to extract information inside the beam profile. Accordingly, it is not possible to increase the resolution inside the beam profile, which is in a plane.
From the foregoing, with the conventional technique of the microscope and the like, it has not been possible not only to increase the resolution inside the beam profile, which is in a plane, but also to observe or measure, in real time, three-dimensional information regarding a biological sample having a thickness such as a cell, in a living state, without damaging the sample.
The present invention is made in view of the above-described background, and an object thereof is to provide an optical distance measuring apparatus having a high resolution in a plane as well as a high resolution with respect to a height or a refractive index distribution outside the plane, and having an effectively high resolution and causing no loss of spatial frequency by accurately reproducing spatial frequency information which an object under measurement has by obtaining a spatial frequency which cannot be obtained with an ordinary imaging optical system.
A first invention of an optical distance measuring apparatus includes:
a light source irradiating a coherent irradiation light;
a scanning element scanning the irradiation light from the light source and sending it to an object under measurement;
a photo detector receiving the irradiation light modulated by being passed through the object under measurement in accordance with the scanning, and performing photoelectric conversion on the irradiation light; and
a measuring unit obtaining phase information of the object under measurement based on a signal photoelectrically converted by the photo detector and a signal to be a reference for the scanning by the scanning element, and obtaining a measurement value regarding the object under measurement based on the phase information.
The operation of the first invention of the optical distance measuring apparatus will be described below.
In the present invention, the coherent irradiation light is irradiated from the light source, and the scanning element scans the irradiation light and sends it, as a scanning beam, to the object under measurement. Further, at least one photo detector receives the irradiation light modulated by being passed through the object under measurement, and performs the photoelectric conversion on the irradiation light. Further, the measuring unit obtains the phase information of the object under measurement based on the signal photoelectrically converted by the photo detector and the signal to be the reference for the scanning by the scanning element, and in accordance with this, it is possible to obtain the measurement value of the optical distance and the like based on the phase information.
Therefore, according to the present invention, it becomes possible to modulate the irradiation light by the scanning of the irradiation light by the scanning element for obtaining an image of the object under measurement, without modulating the irradiation light from the light source by using a special modulation element or separating one irradiation light into lights with two frequencies by using an acoustic optical device. That is, by irradiating the irradiation light to the object under measurement, the irradiation light can be easily modulated without using a special device or element, which enables to realize reduction in cost of the optical distance measuring apparatus.
As a result of the above, in a microscope to which the present invention is applied, it has a quite high in-plane resolution and moreover, an optical distance such as a height or a refractive index distribution regarding an object under measurement can be measured by performing two-dimensional scanning once. For this reason, three-dimensional measurement of a state change or the like of live cells, a micro-machine, or the like can be performed in real time. Specifically, the present invention has large characteristics incomparable to a conventional laser scanning type confocal microscope which obtains two-dimensional information and adds it up in a three-dimensional direction, or the like.
Moreover, when the present invention is applied to a transmitted type microscope, it is possible to perform, in visualization of cells, microorganisms, or the like, observation or measurement of the cells, the microorganisms, or the like in a living state and which are not stained with fluorescence, by using a simple apparatus at high speed and with high resolution. Accordingly, the present invention has large characteristics which are not present in electron microscopes with which cells or the like are inactivated and then measured.
From the foregoing, according to the present invention, it is possible to provide the optical distance measuring apparatus having a high resolution in a plane as well as a high resolution with respect to a height or a refractive index distribution outside the plane, and having an effectively high resolution and causing no loss of spatial frequency by accurately reproducing is spatial frequency information which an object under measurement has by obtaining a spatial frequency which cannot be obtained with an ordinary imaging optical system.
In addition, it is also possible to use the present invention, for education or hobbies, as a microscopic three-dimensional digitizer. For example, by using a three-dimensional printer of nowadays and the present invention in combination, it is possible to easily express progress of cell division or a three-dimensional image of an organ inside a cell of a microorganism as a three-dimensional model, in a living state and without performing processing such as staining.
Further, in the first invention, it is also possible to design such that a direction perpendicular to an optical axis direction of the irradiation light is set as a boundary line, and the photo detector is positioned by being displaced to any one side with respect to the boundary line, and receives the irradiation light passed through the object under measurement.
By designing as above, it is possible to securely obtain sufficient data from the irradiation light, even with one photo detector. Note that the reason why the photo detector is positioned by being displaced to any one side with respect to the boundary line, is because when the photo detector is positioned at a center of the optical axis, phases are reversed across the boundary line, which makes it difficult to obtain sufficient data from the irradiation light.
A second invention of an optical distance measuring apparatus includes:
a light source irradiating a coherent irradiation light;
a scanning element scanning the irradiation light from the light source and sending it to an object under measurement;
two photo detectors existing with a boundary line in a direction perpendicular to an optical axis direction of the irradiation light interposed therebetween, each receiving the irradiation light modulated by being passed through the object under measurement in accordance with the scanning, and performing photoelectric conversion on the irradiation light; and
a measuring unit obtaining phase information of the object under measurement based on signals each photoelectrically converted by each of the two photo detectors and a signal to be a reference for the scanning by the scanning element, and obtaining a measurement value regarding the object under measurement based on the phase information.
If the irradiation light is received by each of the two photo detectors as described above, the one photo detector existing in one side region with respect to the optical axis and the other photo detector existing in a region on the opposite side of the one side region, can respectively receive a scanning beam as amounts whose phases are mutually reversed. Accompanying this, with the use of these photo detectors, it is possible to easily detect an optical distance from a phase difference of the scanning beam. For this reason, when the both photo detectors independently detect the phase differences, and then the measuring unit calculates an average value, it is also possible to obtain data with higher accuracy by reducing an influence of noise and the like.
Further, in the first invention and the second invention, it is also possible to design such that the photo detector is disposed in any region divided by the boundary line along the direction perpendicular to the optical axis direction of the irradiation light and a cross boundary line crossing the boundary line on the optical axis of the irradiation light.
By designing as above, the photo detector is positioned in only any of the divided regions being four divisions in total. Consequently, it becomes possible to employ a photo detector of smaller size and lower cost, and the measuring unit can obtain a required measurement value even with little phase information received by this small-sized photo detector.
On the other hand, in the first invention and the second invention, it can be considered that the scanning element is set to a two-dimensional scanning element scanning the irradiation light in two directions, is respectively, which are orthogonal to each other, and the irradiation light irradiated to the object under measurement by the scanning in at least one direction out of the two directions is modulated.
A third invention of an optical distance measuring apparatus includes:
a light source irradiating a coherent irradiation light;
a scanning element scanning the irradiation light from the light source and sending it to an object under measurement;
a controller connected to the scanning element, and operating an operation of the scanning element to control a scanning speed and a scanning range of the scanning element;
a photo detector receiving the irradiation light modulated by being passed through the object under measurement in accordance with the scanning, and performing photoelectric conversion on the irradiation light; and
a measuring unit obtaining phase information of the object under measurement based on a signal photoelectrically converted by the photo detector and a signal which is issued by the controller and which becomes a reference for the scanning by the scanning element, and obtaining a measurement value regarding the object under measurement based on the phase information.
As described above, by designing such that the controller is connected to the scanning element, and the controller operates the operation of the scanning element to control the scanning speed and the scanning range, it is possible not only to obtain a two-dimensional image but also to perform measurement with an arbitrary modulation amount and in an arbitrary range, only by changing a setting of the controller.
Next, in the first invention to the third invention, it is also possible to design such that the measuring unit extracts a direct-current component and an alternating-current component from the signal photoelectrically converted by the photo detector, calculates, based on a differential signal of the is obtained alternating-current component or a signal as a result of performing Hilbert transform on the obtained alternating-current component, a main frequency component of the modulated signal, and compares this frequency with a spatial frequency which the object under measurement has. Accompanying this, by calculating the main frequency component of the scanned signal, and comparing this frequency with the spatial frequency which the object under measurement has, it becomes possible to correct an MTF value which the optical system has.
Further, in the first invention to the third invention, it is also possible to design such that the measuring unit turns an alternating-current component of the signal photoelectrically converted by the photo detector into digitalized data, and adjusts, by changing an addition amount of the data, a range of obtaining the measurement value regarding the object under measurement.
For example, a signal processing circuit housed in the measuring unit is set to perform, not analog signal processing, but digital signal processing. Further, the measuring unit extracts a direct-current component and an alternating-current component from the modulated signal obtained in accordance with the scanning from the photo detector. In accordance with the extraction of the direct-current component and the alternating-current component as described above, it is possible to detect a frequency of the alternating-current component.
From the foregoing, it is possible to correctly perform quantification of information regarding the optical distance which the object under measurement originally has. Note that the measurement value of the optical distance can be calculated from magnitudes of the direct-current component and the alternating-current component, and a phase signal of the alternating-current component. Further, according to the first invention to the third invention, the main spatial frequency which the object under measurement has can be grasped in a unit of pixel displaying three-dimensional is information, so that band emphasis can be performed not only on the information of the optical distance being the visualized three-dimensional information but also on an arbitrary spatial frequency. For this reason, it is also possible to easily extract a portion, which is desired to be emphasized and observed by an observer, such as a rough structure like a portion with low spatial frequency or a minute structure with high spatial frequency.
Further, a limit of lateral resolution which the optical system has corresponds to an upper limit of detectable frequency, so that it can be considered to sample the alternating-current component and the direct-current component at a frequency which is sufficiently higher than this upper limit frequency and is equal to or more than a frequency corresponding to an optical resolution, when digitalizing the data. Based on the sampled data, by adding data flowed in time-series, it is possible to reduce electrically or optically generated random noise. This consequently leads to improvement of accuracy of measurement data and reduction in noise when displaying a three-dimensional image.
Further, since the scanning speed is constant, by changing the number of data to be added, it also becomes possible to substantially change a range of displaying the image by enlarging or reducing a range of visual field, without changing the optical resolution. Therefore, it also becomes possible to express the range of visual field arbitrarily to a certain extent without substantially changing NA of an objective lens used for irradiation.
Further, if it is set that when the irradiation light passes through the object under measurement, the object under measurement reflects the irradiation light, the photo detector in the first invention receives the reflected light to perform the photoelectric conversion on the light. In this case, by disposing a beam splitter within an optical axis between the light source and the object under measurement, the irradiation light reflected by and returned from the object under measurement can be further reflected by the beam splitter and sent to the photo detector side.
Further, if it is set that when the irradiation light passes through the object under measurement, the irradiation light transmits through the object under measurement, the photo detector in the first invention disposed on the optical axis, for example, receives the transmitted light and performs the photoelectric conversion on the light.
As described above, in the optical distance measuring apparatus of the present invention, the coherent irradiation light is irradiated from the light source, and the scanning element scans the irradiation light and sends the light, as the scanning beam, to the object under measurement, thereby modulating the light. Further, one photo detector receives the irradiation light passed through the object under measurement, and performs the photoelectric conversion on the light. Therefore, the excellent effect is exhibited such that in accordance with the obtainment of the phase information of the object under measurement performed by the measuring unit based on the signal photoelectrically converted by the photo detector and the signal to be the reference for the scanning by the scanning element, it becomes possible to perform quantitative calculation of the optical distance and the like.